Simultaneous transmit and receive (STAR) refers to the ability of a radio frequency (RF) circuit, device, or system to transmit and receive at the same time, in the same frequency band, with adequate performance in the receiver. Such capability is desired for applications such as, for example, cognitive radio, full-duplex communications systems and relays, spectral management and frequency reuse, multi-function, multi-beam radar and “look through” radar jammers. In a conventional approach, RF transmit and receive operations within a particular frequency band are performed at different times. This is because transmit energy from a transmit antenna will typically leak into the front end of a collocated receiver and overdrive the receiver if transmit and receive operations are performed concurrently. This transmitter leakage can mask the desired receive signals, thus making it difficult or impossible to detect, demodulate, and decode the signals. For STAR operation to be possible, therefore, a certain minimum level of isolation must be maintained between a transmit antenna and a receive antenna.
Various cancellation schemes can be used to improve transmit and receive isolation in a STAR system. For example, some STAR systems inject a filtered version of the transmit signal into the receiver in order to cancel the transmit signal at one or more of the RF signal, the intermediate frequency (IF) or the digital baseband stage of the receiver. Performance of such systems is limited by noise, dynamic range, channel knowledge, and linearity of the receiver and cancelling components. Further, to achieve adequate isolation, multiple layers of cancelling might be required. Another cancellation scheme is spatial cancelling that actively controls the system antenna pattern to increase transmit and receive isolation. Spatial cancellation reduces the transmit signal before it reaches the receive antenna, resulting in higher receiver linearity and dynamic range, and improving the effectiveness of other cancelling techniques. Spatial cancellation allows stronger transmitters and more sensitive receivers to operate simultaneously.
Further, STAR systems might employ high isolation antennas where the transmit and receive antenna elements are in a fixed configuration that has high intrinsic antenna isolation. However, this provides only minimal antenna pattern control and, thus, is not feasible for directive arrays. In other systems, adaptive beamforming might be performed where each element in the transmit and receive antenna arrays has an adaptive filter. This is commonly implemented digitally, with every element having a dedicated digital channel (i.e. digital beamforming), although analog implementations are also possible, for example by using physical tapped delay lines. Although such a system provides robust control of radiated power over space, time and frequency (Space-Time Adaptive Processing or “STAP”), such a system is very expensive.